Osmosis 3.1 and 3.2

Question 1

What is osmosis?

Answer 1

Net movement of H2O from regions of high H2O concentration to regions of low H2O concentration

Question 2

What are the differences between Osmosis and diffusion?

Answer 2

Net movement of SOLUTE from regions of high SOLUTE concentration to regions of low SOLUTE concentration

Water can move freely between cells and the ECF so that the body is in osmotic equilibrium. Not all solutes move freely.

ALL H2O movements in the body are passive, (via aquaporins, protein water channels).

H2O concentration is inversely related to the concentration of solute, ie the more solute particles there are in solution, the more they will displace H2O molecules lowering the concentration of H2O.

Question 3

Diffusion through a freely permeable membrane , water and glucose example?

Answer 3

If a solution of different concentration is separated by a membrane permeable to the H2O and solute then after a time will end up with equal concentration and equal volume either side of membrane = DIFFUSION!

Question 4

Diffusion through a selective membrane, water and glucose example?

Answer 4

If a solution of different concentration is separated by a membrane permeable to H2O only, then after a time will end up with equal concentration either sided of membrane but different volumes. Assumes compartments are expandable.

Question 5

What is osmotic pressure?

Answer 5

If we try to oppose this increase in volume the pressure required to do so is known as the osmotic pressure = the P required to prevent water movement

Question 6

When is it diffusion and when is it osmosis?

Answer 6

Where we have diffusion we also have osmosis (water and solute move, in opposite directions).
Where we have osmosis, we may or may not have diffusion – if the membrane is only permeable to water, then water moves but the solute does not (so no diffusion).
Osmosis with no diffusion will result in a change in cell volume!

Question 7

Why are cell membranes semi permeable membranes?

Answer 7

Cell membranes act as semipermeable membranes. They are permeable to H2O and gases but some molecules in the ECF and ICF are unable to cross the membrane.

Question 8

Which wall is permeable to ions?

Answer 8

Ions pass freely across the capillary wall so exchange readily between plasma and ISF but they do not penetrate the cell membrane hence differential distribution between ECF and ICF.

Question 9

What is concentration measured in?

Answer 9

concentrations are expressed as molarity or mol/l and a mole consists of 6.02 x 1023 molecules.

Question 10

What determines the osmotic effect on H20?

Answer 10

However it is the number of solute particles, NOT molecules, which determine the osmotic effect on [H2O] and this can be misleading because some molecules dissociate in solution.

Question 11

What does osmolarity measure?

Answer 11

measures the concentration of biological solutions in units of “OSMOLES” and describes the number of particles/L of solution, (in reality use milliosmoles, because biological solutions are so weak).

Question 12

What is the osmolarity of normal human plasma and within cells?

Answer 12

285 mOsmol/l
-often taken as 300

Question 13

What does osmolarity tell us about the particles?

Answer 13

Osmolarity ONLY describes the number of particles in solution, it says nothing about the NATURE of the particles, critically it does not tell us if the particles can cross cell membranes.

Question 14

What is an osmotic flux?

Answer 14

If a solute cannot cross the membrane, any change in its concentration produces an osmotic flux, causing a net movement of water in one direction or the other, which changes the cell volume.

Question 15

What does the volume of a cell always depend on?

Answer 15

the volume of a cell at any time is dependent on the concentration of non-penetrating solutes on the either side of the membrane.

Question 16

What are the non penetrating solutes in the ECF and ICF?

Answer 16

In ECF, Na+ and Cl- act as non-penetrating solutes.
In ICF, K+ (and organic anions) act as non-penetrating solutes

Question 17

What does osmolarity describe?

Answer 17

Osmolarity describes total number of particles in solution

Question 18

What does tonicity describe?

Answer 18

Tonicity describes the number of non-penetrating particles in solution

Question 19

Why is tonicity a more important term?

Answer 19

Determines cell volume

Question 20

What is an isosmotic solution?

Answer 20

An isosmotic solution has the same total number of solute particles as normal ECF (plasma).

Question 21

What is a hypo-osmotic solution?

Answer 21

Solutions with fewer total solute particles than plasma are hypo-osmotic.

Question 22

What is a hyper osmotic solution?

Answer 22

Solutions with greater number of total solute particles than plasma are hyper-osmotic

Question 23

What is an isotonic solution?

Answer 23

An isotonic solution has the same number of non-penetrating solute particles as normal ECF (plasma).

Question 24

What is a hypotonic solution?

Answer 24

Solutions with fewer non-penetrating solute particles than plasma are hypotonic.

Question 25

What is a hypertonic solution?

Answer 25

Solutions with greater non-penetrating solute particles than plasma are hypertonic.

Question 26

Why is the osmolarity of normal human plasma the same as within cells?

Answer 26

has to be the same because there are no barriers to movement of H20.
Means the number of particles inside the cell = the number of particles outside the cell, per unit volume, BUT the composition of particles (non-penetrating/ion species) differs vastly.

Question 27

How do we predict tonicity and cell volume?

Answer 27

Need to know the composition of that osmolarity

Question 28

Why do cells in hypotonic solutions swell?

Answer 28

-Fewer non penetrating particles than normal
-more water in ECF
-water enters down a chemical gradient

Question 29

Why do cells in hypertonic solutions shrink?

Answer 29

–more non-penetrating particles than normal
-less water in ECF
-water leaves down a chemical gradient

Question 30

What is intravascular haemolysis?

Answer 30

Cell bursting

Question 31

Why is cell bursting a problem?

Answer 31

Lysed (burst) cells introduce protein to ISF, increasing tonicity of ECF in an uncontrolled manner, making management very complicated.

Question 32

What will happen if RBC placed in water?

Answer 32

RBC placed in water will swell and burst because water moves in by osmosis (due to presence of non-penetrating ions in the ICF)

Question 33

What will happen if you place RBC’s in hyperosmotic aqueous urea solution?

Answer 33

RBC placed in hyperosmotic urea will still swell and burst.
This is because urea is a penetrating particle and because the urea is in an aqueous hypotonic solution
Because urea is a penetrating particle it enters the cell until equilibrium is reached (equal numbers of urea particles inside and out).
However, there are lots of other things inside cells in addition to urea, so the water concentration remains greater outside than inside. This causes water to enter by osmosis.

Question 34

What will happen to the volume of red blood cells in patients with ureamia (excess urea in their plasma)?

Answer 34

Very little!
Difference is in vivo there is lots of NaCl in the ECF and this is isotonicso when you add urea to an iso-tonic solution, the urea equilibrates across the membrane, but the resulting ECF remains iso-tonic, so no net movement of particles occurs. The ICF and ECF osmolarity has changed by the same amount of penetrating particle, but the non-penetrating particles remain unchanged so the ECF remains isotonic (despite now being hyper-osmoticIn the earlier in vitro example, in an aqueous solution, the urea is accumulating in a hypo-tonic solution (water), so once equilibrated the solution is still hypotonic so water moves into the cell. Even a hyperosmotic aqueous solution of urea will be hypotonic and cause cells to swell. In effect the urea makes no difference to movement of water across membranes.